The sintering of (Ca_0.6,Mg_0.4)Zr₄(PO₄)₆ powder (CMZP) synthesized via a new solid-state reaction method was investigated for application as a thermal barrier in next generation internal combustion engines. Specifically, CMZP is being considered as a potential material for the manufacture of exhaust port liners to increase the overall performance of diesel engines. The effects of firing time, firing temperature, and amount of ZnO sintering aid on modulus of rupture (MOR), bulk density, and coefficient of thermal expansion (CTE) were determined for both dry pressed and slip cast samples to optimize the physical properties for this application. For both processing methods, suppressing the formation of secondary interparticle phases (grain boundaries) was found to be the controlling factor for obtaining high strength and positive thermal expansion.

For a given ZnO level, increases in firing time and/or temperature resulted in improved density but a degradation in microstructure (undesirable grain growth, formation of a liquid phase, and intra-/trans- granular microcracking), accompanied by a reduction in MOR and negative CTE values. Therefore, optimizing bulk density was determined to be counter-productive for improving strength when ZnO is used as a sintering aid.